1. Field of the Invention
The present invention generally relates to a Pulse-Doppler radar apparatus outputting a pulse wave, receiving a reflected radio wave, and measuring a distance to and/or a speed of a target generating the reflected wave, based on a difference in frequency between the pulse wave and the reflected wave, that is, based on a baseband signal containing a beat frequency component. More particularly, the invention relates to improvements in detection precision of a millimeter wave Pulse-Doppler radar apparatus installed in a mobile object such as an automobile for using a pulse wave in a millimeter band to detect a distance to or a relative speed of a target such as a person, automobile or obstacle around the mobile object.
2. Description of the Related Art
FIG. 7 is a block diagram showing a construction of a related-art Pulse-Doppler radar apparatus or FM pulse Doppler radar apparatus installed in an automobile. Referring to FIG. 7, reference numeral 1 indicates an oscillator for outputting a high-frequency signal corresponding to a preset voltage; 16 indicates a first switch directly connected to an output of the oscillator and switching between outputs of the high-frequency signal; 4 indicates a transmitter amplifier constituting one of the output destinations of the switch 16 and amplifying the high-frequency signal; indicates an antenna; 6 indicates a receiver amplifier for outputting a received signal; 9 indicates a second harmonic mixer constituting the other output destination of the switch 16 and outputting a baseband signal corresponding to a difference between the frequency of the received signal and-that of the high-frequency signal; 10 indicates a signal processing part for controlling the preset voltage to the oscillator 1, and for measuring a distance to and/or speed of a target generating the reflected radio wave, based on the baseband signal. Numeral 17 indicates a second switch for selectively connecting the transmitter amplifier 4 or the receiver amplifier 6 to the antenna 5.
A description will now given of the operation according to the related art.
The signal processing part 10 outputs a preset voltage corresponding to a predetermined oscillation frequency to the oscillator 1 so that the oscillator outputs a high-frequency signal corresponding to the preset voltage. When the first switch 16 and the second switch 17 are operated so as to select the transmitter amplifier 4 in this state, a pulse wave based on the amplified high-frequency signal is output from the antenna 5 while the transmitter amplifier 4 remains selected.
When the first switch 16 and the second switch 17 are operated to select the receiver amplifier 6, a signal derived from a radio wave received by the antenna 5, for example, the radio wave reflected by a target of the pulse wave, is input to the receiver amplifier 6. The second harmonic mixer 9 mixes the high-frequency signal with the output of the receiver amplifier 6 so as to produce a baseband signal. The signal processing part 10 measures a distance to and/or a speed of the target generating the reflected wave, based on waveforms of beat frequency components contained in baseband signals obtained in a plurality of detecting processes.
FIGS. 8A-8E are waveform charts generated in a pulse Doppler apparatus according to the related art. FIG. 8A shows a high-frequency signal in a millimeter band output from the oscillator 1; FIG. 8B is a signal output from the transmitter amplifier 4; FIG. 8C shows a locally generated signal output from the first switch 16 to the second harmonic mixer 9; FIG. 8D shows a received signal output from the receiver amplifier 6 to the second harmonic mixer 9; and FIG. 8E shows a baseband signal (video signal) output from the second harmonic mixer 9 to the signal processing part 10.
FIGS. 9A and 9B illustrate the principle of measuring a distance and speed according to the related-art Pulse-Doppler apparatus. Referring to FIGS. 9A and 9B, time is plotted horizontally and frequency is plotted vertically. Curve d indicates a waveform of a transmitted signal; curve e indicates a waveform of a received signal; and curve f indicates a waveform of the video signal frequency produced when the frequency of the baseband signal (video signal) shown in FIG. 8E is plotted on the time axis. xcex94F indicates a modulation frequency component of the pulse wave (the transmitted signal shown in FIG. 8B), and Fb indicates a frequency component, called a beat frequency component, of the baseband signal (video signal). In order to simplify the illustration, FIGS. 9A and 9B show waveforms having the relative speed of zero.
By repeating measurements of the pulse waves during a period of a lump of a prescribed number of pulses of the high-frequency signal (pulse wave), data of the curve f for a period of the lump is obtained. Based on a waveform of the curve f, the signal processing part 10 can measure the distance to and the speed of the target generating the reflected wave.
FIG. 10 is a block diagram showing a construction of the related-art Pulse-Doppler radar apparatus in which improvement is made. This construction is disclosed in, for example, xe2x80x9cIEEE MTT-S Digest, pp.227-230 (1998)xe2x80x9d and in xe2x80x9cEuropean Microwave Conference Amsterdam, pp619-629 and 630-635 (1998)xe2x80x9d. Referring to FIG. 10, numeral 18 indicates a frequency-multiplier disposed between the first switch 16 and the transmitter amplifier 4. With this construction, the oscillation frequency of the oscillator 1 may be half that of the output frequency, which is useful particularly when the high-frequency signal in a millimeter band is used.
In the above-described construction of the related-art Pulse-Doppler radar apparatus, the load impedance of the oscillator 1 varies to produce a momentary open state when the first switch 16 is operated for switching. As a result of this variation in the load, the oscillation frequency of the oscillator 1 varies. FIG. 11 is a graph showing the relationship between the load impedance of the oscillator 1 and the oscillation frequency under the condition of a constant preset voltage. Referring to FIG. 11, the load impedance is plotted horizontally and the oscillation frequency is plotted vertically. FIG. 11 shows that, when the load impedance of the oscillator 1 varies, for example, from 50 xcexa9 to a momentary open-state level, such a load variation causes the oscillation frequency of the oscillator 1 to vary significantly.
As a result of this, a variation in the oscillation frequency resulting from switching is included in the pulse wave so that the beat frequency component obtained at each point along the time line varies. It leads to disturbance in the waveform of the beat frequency component obtained a plurality of measurements. Thus, the precision in measurement of the distance and speed cannot be improved beyond a certain level.
Accordingly, a general object of the present invention is to provide a Pulse-Doppler radar apparatus in which the aforementioned problem is eliminated.
Another and more specific object of the present invention is to provide a Pulse-Doppler radar apparatus in which the load impedance variation of the oscillator is appropriately controlled so that the distance to and/or speed of a target can be measured with high precision incapable in the related art.
The aforementioned objects are achieved by a Pulse-Doppler radar apparatus comprising: an antenna; an oscillator for outputting a high-frequency signal corresponding to a preset voltage; a distributor connected to an output of the oscillator so as to divide the high-frequency signal into a first distributed signal and a second distributed signal; a first mixer supplied at one of its two input terminals with the first distributed signal and outputting a transmitted signal of twice the frequency as that of the first distributed signal on application of a dc voltage to the other input terminal; a transmission and reception unit for outputting a pulse wave in response to the transmitted signal via the antenna and outputting a received signal based on the corresponding reflected wave received by the antenna; a second harmonic mixer supplied at one of its two input terminals with the second distributed signal, supplied at the other input terminal with the received signal, and outputting a baseband signal based on the second distributed signal and the received signal; and a signal processing unit outputting the preset voltage to the oscillator, outputting the dc voltage to the first harmonic mixer, and measuring a distance to and/or speed of a target generating the reflected wave, based on the baseband signal.
The aforementioned objects can also be achieved by a Pulse-Doppler radar apparatus comprising: an antenna; an oscillator for outputting a high-frequency signal corresponding to a preset voltage; a distributor connected to an output of the oscillator so as to divide the high-frequency signal into a-first distributed signal and a second distributed signal; a multiplier for receiving the first distributed signal and outputting a transmitted signal having twice the frequency as that of the first distributed signal; a transmission and reception unit for outputting a pulse wave in response to the transmitted signal via the antenna, and outputting a received signal based on the corresponding reflected wave received by the antenna; a second harmonic mixer supplied at one of its two input terminals with the second distributed signal and, at the other input terminal, with the received signal, and for outputting a baseband signal based on the second distributed signal and the received signal; and a signal processing unit outputting the preset voltage to the oscillator, and measuring a distance to and/or speed of a target generating the reflected wave based on the baseband signal.
The transmission unit may comprise: a transmitter amplifier for amplifying the transmitted signal and supplying the same to the antenna; a receiver amplifier for amplifying a signal derived from the reflected wave received by the antenna and outputting the corresponding received signal; and a switch for selectively connecting one of the transmitter amplifier and receiver amplifier, to the antenna, wherein the signal processing unit controls operation of the switch.
The transmission and reception unit may comprise: a transmitter amplifier for amplifying the transmitted signal and supplying the same to the antenna; a receiver amplifier for amplifying a signal derived from the reflected wave received by the antenna and outputting the corresponding received signal; and a circulator for selectively connecting one of the transmitter amplifier and receiver amplifier, to the antenna.
The transmission and reception unit may comprise: an amplifier for amplifying an input signal; a two-input switch connected to an input of the amplifier and having two input terminals, one of the input terminals receiving the transmitted signal: a two-output switch connected to an output of the amplifier and having two output terminals, one of the output terminals being connected to the second harmonic mixer; and a circulator connected to the other output terminal of the two-output switch, the other input terminal of the two-input switch and the antenna, and selectively connecting the antenna to one of: the other output terminal of the two-output switch; and the other input terminal of the two-input switch, wherein the signal processing unit controls operation of the two-input switch and two-output switch.
The signal processing unit may change the preset voltage for the oscillator with time so that the oscillator outputs a frequency-modulated high-frequency signal with its frequency varying in accordance with the preset voltage.